1. Field of the Invention
The invention relates to the heating of fluidized beds. More particularly, it relates to an improved process for the heating of such beds and for enhancing the fluidized bed conversion of silane to polycrystalline silicon.
2. Description of the Prior Art
A variety of means are well known in the art for applying necessary heat to fluidized bed reaction zones. A suitable heat transfer fluid and inductive or electrical resistance heaters are examples of such means. While adequate for the purposes of many applications, such means are not entirely satisfactory for other fluidized bed applications because of the particular nature of the desired reactions occurring therein and of undesirable side effects that accompany such reactions using fluidized beds heated by conventional means.
The production of polycrystalline silicon from silane or a halo-silane in a fluidized bed reaction zone is a significant example of the limitations of conventional means for heating such fluidized beds. In this example, silicon seed particles are suspended in a fluidizing gas stream into which, for example, silane is injected. Process conditions are desirably maintained so that the decomposition of the silane occurs in the heterogeneous region or manner, i.e., the silane decomposes on the surface of the seed particles in the fluidized bed. In this manner, the seed particles grow by the deposit of silicon thereon so that sufficiently large particles of silicon product are grown to permit convenient removal thereof from the reaction zone, while by-product hydrogen can be separately removed as overhead from said reaction zone. When chlorosilanes are decomposed, the by-product gas will comprise HCl which is more difficult to handle than the hydrogen by-product of silane decomposition.
Conventional means of heating a fluidized bed reaction zone will result, however, in an undesired coating of silicon on the wall of the reaction zone, possibly in preference to the desired fluidized seed particle growth. Silane decomposition in fluidized beds employing conventional heating means likewise results in the homogeneous decomposition of silane to form fine silicon powder or dust. This large surface area, light, fluffy powder is undesirable in fluidized bed operations as it comprises waste material or requires careful and costly additional handling for recovery and consolidation or melting without unacceptable contamination due to environmental effects.
The development of an improved process for heating fluidized bed reaction zones is, therefore, desirable and of great significance to the development of low-cost silicon technology. In the production of high purity polycrystalline silicon, current commercial technology constitutes a low volume, batch operation generally referred to as the Siemens process. This technology is carried out in the controlled atmosphere of a quartz bell jar reactor that contains silicon rods electrically heated to about 1100.degree. C. Trichlorosilane, in concentrations of less than 10% in hydrogen, is fed to the reactor under conditions of gas flow rate, composition, silicon rod temperature and bell jar temperature adjusted so as to promote the heterogeneous decomposition of the chlorosilane on the substrate rod surfaces. A general description of the Siemens-type process can be found in the Dietz, et al. patent, U.S. Pat. No. 3,979,490.
Because of the inherent limitations of such batch processing and because of the relatively high processing costs associated with the commonly employed process for reacting metallurgical grade silicon with HCl to form trichlorosilane, polycrystalline semiconductor grade silicon made from metallurgical grade silicon costing about $0.50/lb. will cost on the order of about $30/lb. and up. In growing a single crystal of this semiconductor grade material, the ends of the single crystal ingot are cut off, and the ingot is sawed, etched and polished to produce polished wafers, as for solar cell application, with mechanical breakage and electronic imperfection reducing the amount of usable material obtained. As a result, less than 20% of the original polycrystalline, semiconductor grade silicon will commonly be recoverable in the form of useful wafers of single crystal material. The overall cost of such usable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a significant factor in the overall economics of such applications. It will be understood that such material costs are also of significant concern in applications of such high purity, single crystal silicon for various semiconductor applications apart from use in solar cell structures.
The economic feasibility of utilizing silicon for solar cell and for semiconductor applications would be enhanced, therefore, if the overall cost of producing high purity, single crystal silicon in desired form could be reduced. One important area of interest in this regard is in the production of polycrystalline silicon from silane, chlorosilanes or other halo-silanes in a fluidized bed reaction zone as discussed above. The decomposition of such silanes in a fluidized bed reaction zone is disclosed in Ling, U.S. Pat. No. 3,012,861 and Bertrand, et al., U.S. Pat. No. 3,012,862. In this approach, a silicon-containing gas is injected into a reaction chamber containing particles of elemental silicon small enough to be fluidized and maintained in ebullient motion to expose their entire surfaces for nucleating contact with the silicon-containing gas. The reaction chamber and the fluidized bed of silicon particles are maintained at a temperature within the thermal decomposition range of the gas and below the melting point of silicon. In the Ling patent, the use of external heating means 11, such as electric resistance heaters, surrounding the vertical walls of reactor 1 is disclosed. Bertrand, et al. disclose electrical or other type of external heating means 2, with electrical resistance heating elements said to be preferred in small scale operations and other heating means, such as indirect gas firing can be used in large scale operations. The preheating of hydrogen and/or other reactants prior to introduction into the reactor so that little or no additional heat need be supplied through the wall of the reaction zone is also taught by Bertrand, et al.
The silicon-containing compound injected into the reaction chamber, particularly silane, will be subject to homogeneous decomposition upon exposure to the reaction conditions within the chamber as well as the desired heterogeneous decomposition and deposition of product silicon on the seed particles present in the fluidized bed. As a result of such homogeneous decomposition, considerable quantities of silicon dust are formed. This dust is undesired in the fluidized bed process, as noted above, as it results in a considerable loss of material and/or additional processing expense. Such undesired dust formation is a factor that has heretofore deterred the development of the fluidized bed approach as a practical alternative to the conventional Siemens process. The need continues, therefore, for the development of technically and economically feasible alternatives to the Siemens process for the production of high purity silicon for semiconductor and solar cell applications.
It is another object of the invention to provide an improved process for the heating of fluidized beds.
It is another object of the invention to provide a process enhancing the heterogeneous decomposition of the feed gas passed to a heated fluidized bed reaction zone.
It is another object of the invention, therefore, to provide an improved process and apparatus for the production of low-cost, high purity polycrystalline silicon.
It is another object of the invention to provide a process and apparatus for the enhanced production of high purity silicon on a continuous or semicontinuous basis.
It is another object of the invention to provide an improved process for the production of silicon capable of advantageously employing silane as the silicon-containing feed material.
It is a further object of the invention to provide an improved process and apparatus for the fluidized bed decomposition of silane and halo-silanes with minimal formation of undesired silicon dust.
It is a further object of the invention to provide an improved process and apparatus for the production, at relatively high production rates, of high purity polycrystalline silicon suitable for semiconductor and solar cell applications.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.